Acknowledge back pager with apparatus for controlling transmit frequency

ABSTRACT

An acknowledge back (ack-back) pager is provided for use in a paging system including a central station which transmits a group of message signals to a group of ack-back pagers which are addressed as a group. The users of the group of addressed ack-back pagers indicate a response to their respective pagers thus providing ack-back data. The pagers in the group of addressed ack-back pagers then simultaneously transmit back to the central station their ack-back data on different frequency sub-bands, a different frequency sub-band being allocated to each of the pagers in the group. To accurately control the ack-back transmit frequency, the pager receives and down-converts the F RX  signal thus producing a down-converted F C  signal, the frequency of which is measured and stored. The pager selects which sub-band of a plurality of frequency sub-bands within a predetermined range of frequencies is to be used for transmission of the ack-back signal. The pager determines a frequency offset F D  corresponding to the selected sub-band, such offset being with respect to a predetermined frequency within such range of frequencies. The pager generates an acknowledge back signal at a frequency (F C  +F D )-F CTX  and up-converts such signal to a transmit frequency F TX  corresponding to said selected sub-band, F CTX  being the frequency of a down-converted sample of the F TX  signal.

BACKGROUND OF THE INVENTION

This invention relates in general to radio communications systems. Moreparticularly, the invention relates to radio paging systems.

In the past several years, radio paging technology has advanced from therather simple tone-only pager (tone alert only, no voice), to the toneand voice pager (tone alert with a voice message) and more recently tothe alphanumeric display pager. In a typical conventional alphanumericdisplay paging system such as that shown as system 10 in FIG. 1, acentral transmitter or paging terminal 20 is used to generate the radiopages which are transmitted via a radio link to a fleet of pagingreceivers 1, 2, 3 . . . N, wherein N is the total number of pagers insystem 10. A unique digital address is associated with each of pagingreceivers 1, 2, 3 . . . N. A page which is transmitted by pagingterminal 20 consists of the unique digitally encoded address of theparticular pager to which the page is targeted, immediately followed bya corresponding digitally encoded numeric or alphanumeric page messagewhich is intended for display on the target pager.

Typically, the numeric or alphanumeric page message is stored in amemory within the paging receiver for later recall and display by thepager user. Paging receivers are available with a wide range of messagestorage capabilities which range from the ability to store just a fewrather short numeric page messages to the ability to store a relativelylarge number of longer alphanumeric page messages.

However, conventional display paging systems are generally one waysystems. That is, the user receives a paging message from the centralterminal but has no way of responding to that message with his or herpager. Instead, the pager user must seek out a telephone or other meansof responding to the originator of the paging message.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention is to provide anacknowledge back (ack-back) pager which is capable of responding back tothe paging terminal and the caller.

Another object of the present invention is to provide an ack-back pagerwhereby a group of addressed ack-back pagers are capable ofsimultaneously transmitting acknowledge back signals on a plurality ofrespective predetermined sub-band frequencies.

Another object of the invention is to provide an acknowledge back pagerwhich can selectably transmit acknowledge back signals on any one of theaforementioned sub-band frequencies with high accuracy.

In one embodiment, the acknowledge back pager of the invention isemployed in a radio paging system including a central paging station fortransmitting paging signals on a paging channel frequency F_(RX) to aplurality of remotely located acknowledge back radio pagers. The centralstation transmits a reference carrier signal, that is an F_(RX)reference signal, at frequency FRX at selected times. The acknowledgeback pager is capable of controlling the frequency F_(TX) at which theacknowledge back pager transmits acknowledge back signals. The pagerincludes a receiver for receiving the F_(RX) reference signal. The pagerfurther includes a measuring or counting circuit, coupled to thereceiving means, for measuring the frequency of the F_(RX) signal. Thepager also includes a selecting circuit for selecting a sub-band from aplurality of frequency sub-bands within a predetermined range offrequencies thus determining a selected sub-band for transmission of anack-back signal. The pager includes a determining circuit fordetermining a frequency offset F_(D) corresponding to the selectedsub-band, the offset F_(D) being with respect to a predeterminedfrequency within the range of frequencies. A transmitter is coupled tothe determining circuit for generating an acknowledge back radiofrequency signal at a frequency F_(RX) +F_(D).

The features of the invention believed to be novel are specifically setforth in the appended claims. However, the invention itself, both as toits structure and method of operation, may best be understood byreferring to the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional display type radio pagingsystem.

FIG. 2 is a block diagram of the ack-back paging system of the presentinvention.

FIG. 3 is a block diagram of the central station employed in the pagingsystem of FIG. 2.

FIG. 4A is a time vs. event representation of the transmissions from thecentral station of the system of the invention.

FIG. 4B is a representation of an address block used in the pagingprotocol of the paging system of the invention.

FIG. 4C is a representation of a message block used in the pagingprotocol of the paging system of the invention.

FIG. 4D is a time vs. event representation of the receiver portion ofthe central station.

FIG. 4E is a time vs. event representation of the activity of ack-backpager AB-1.

FIG. 4F is a time vs. event representation of the activity of ack-backpager AB-2.

FIG. 4G is a time vs. event representation of the activity of ack-backpager AB-M.

FIG. 4H is a time vs. event representation of the activity of a nonack-back pager in the paging system of the invention.

FIG. 4I is a time vs. event representation of the activity of an unpagedack-back pager in the paging system of the invention.

FIG. 5 is a flowchart depicting the operation of the central station inthe paging system of the invention.

FIG. 6 is a block diagram of one of the ack-back pagers employed in thepaging system of the invention.

FIG. 7 is subchannel frequency look-up table employed by the ack-backpagers in the system of the invention.

FIGS. 8A and 8B in combination depict a flowchart of the operation ofthe ack-back pagers of the paging system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a simplified block diagram of the acknowledge back pagingsystem 100 of the present invention. Paging system 100 includes acentral station or paging terminal 110 which is capable of bothtransmitting outgoing paging signals and of receiving acknowledge back(ack-back) paging signals. Paging system 100 includes a plurality ofack-back pagers 121, 122 . . . P, wherein P is the total number ofack-back pagers in the pager population of system 100. Each of ack-backpagers 121, 122 . . . P has the capability of receiving paging signalsfrom central station 110 and of permitting the pager user, and/or pager,to respond to such paging signals. That is, pagers 121, 122 . . . Ppermit the user to reply or acknowledge back to a page from centralstation 110. It is noted that conventional non ack-back pagers such aspager 130 are also includable in system 100. In FIG. 2, double arrowsbetween central station 110 and each of ack-back pagers 121, 122 . . . Pare used to denote that two way communication exists between centralstation 110 and such ack-back pagers. A single arrow denotes that onlyone way communication exists between station 110 and pager 130.

It is noted that in one embodiment of the invention, in an acknowledgeback paging system, acknowledge back pagers reply back with acknowledgeback signals at an approximately 2 watt power level. Since, in thisexample, the power level of the ack-back signal is approximately 20 dBsmaller than the power radiated by the paging transmitter, the bandwidthof the ack-back signal is kept small (approximately 100 bits/sec in thisembodiment) in order to keep the range of the central station and theacknowledge back pager approximately equal. The relatively narrowbandwidth of the ack-back signal imposed a stringent frequency tolerance(approximately 30 Hz in this embodiment) on the transmitter portion ofthe ack-back pager.

FIG. 3 is a more detailed block diagram of central station or pagingterminal 110. Central station 110 includes a conventional telephoneinterface 140 of the type generally used for central paging terminals.Telephone interface 140 couples outside telephone lines 141, 142, etc.to an input 150A of a microcomputer 150. Telephone interface 140converts message signals from lines 141, 142, etc. to digital signalswhich microcomputer 150 can process. For example, a caller wishing tosend an alphanumeric page to an ack-back pager user uses dual tone multifrequency (DTMF) to key in a desired message. Telephone interface 140then converts such analog DTMF alphanumeric message to its digitalequivalent which microcomputer 150 processes as discussed later in moredetail. Central station 110 further includes a keyboard 160 coupled to adata input 150B of microcomputer 150. Keyboard 160 permits an operatorto directly input messages into microcomputer 150 for transmission topagers within the pager population.

A read only memory (ROM) 170 is coupled to a memory port 150C ofmicrocomputer 150. ROM 170 includes a control program which controls theoperation of microcomputer 150 and the circuits coupled thereto. Arandom access memory (RAM) 180 is coupled to a memory port 150Dmicrocomputer 150. RAM 180 provides temporary storage space formicrocomputer 150 as it carries out the instructions of the controlprogram within ROM 170.

When a paging message and the identity of the particular pager to beaddressed are provided to microcomputer 150, the control program causesmicrocomputer 150 to generate digital paging signals at its output 150Eaccording to the protocol later described. Microcomputer output 150E iscoupled via a level shifter 190 to the input of a transmitter 200. Theoutput of transmitter 200 is coupled to an antenna 210 having dimensionsand characteristics appropriate to the particular paging frequencychannel selected for the operation of central station 110. Level shifter190 serves to adjust the signal level of the paging signals generated atmicrocomputer output 150E to a level appropriate for the input oftransmitter 200.

For purposes of this example, it will be assumed that ack-back pagers121, 122-P are acknowledging back via phase shift keyed (PSK) digitalmodulation. Those skilled in the art will appreciate that other forms ofmodulation as well may be employed by acknowledge back pagers 121, 122-Pto respond to the paging signals transmitted by central station 110. Insuch a PSK embodiment, central station 110 includes a receive antenna220 for receiving the ack-back signals transmitted by ack-back pagers121, 122-P. In actual practice, antenna 210 may also be employed asantenna 220. Receive antenna 220 is coupled to the input of a PSKreceiver 230 which includes an in-phase (I) output 230A and a quadrature(Q) output 230B. Receiver outputs 230A and 230B are respectivelyconnected to inputs 240A and 240B of digital signal processor 240. Onedigital signal processor which may be employed as processor 240 is themodel DSP56000 manufactured by Motorola, Inc. Digital signal processor240 includes a control input 240C which is coupled to a control output150F of microcomputer 150 to permit microcomputer 150 to controlprocessor 240. Digital signal processor 240 further includes a dataoutput 240D which is coupled to the data input 150G of microcomputer150. Thus, it is seen that digital signal processor 240 decodes thedigital data received at the I and Q inputs 240A and 240B thereof andtransforms such information into digital data which is provided tomicrocomputer data input 150G.

FIG.'s 4A-4I are timing diagrams which show the signaling protocolemployed by central station 110 and ack-back pagers 121, 122-P. Morespecifically, FIG. 4A is a simplified timing diagram of the pagingprotocol transmitted by central station 110. In FIG. 4A, time isrepresented on the horizontal axis and respective events are denoted asthey occur at designated points in time along such time axis. Centralstation 110 first transmits a preamble signal 300 during a time intervalT1. In one embodiment, preamble symbol 300 consists of a plurality ofalternating 0's and 1's transmitted for a duration of time T1. Forexample, preamble symbol is a 010101 . . . signal.

In accordance with the present invention, central station 110 groupspaging addresses into groups of M wherein M is the number of pagingaddresses in a particular group. For purposes of this example, and notby way of limitation, the number of paging addresses and thus the numberof messages corresponding to such addresses is selected to be 20 (thatis, M=20). That is, as messages are called into central station 110 viatelephone interface 140 or keyboard 160, such paging messages andcorresponding address information are held or stored in RAM 180 until agroup of up to M=20 messages has been provided to station 110. Inalternative embodiments of the invention, non ack-back pages may beinterspersed with ack-back pages to increase the efficient throughput ofthe paging system if desired as will be discussed later. The group ofM=20 ack-back pagers is a subgroup of the overall population of Ppagers. Once station 110 has received 20 or M paging messages,microcomputer 150 sequentially transmits the 20 corresponding addressesas a group 310 during a time interval T2 subsequent to time interval T1as shown in FIG. 4A.

FIG. 4B shows the sequential relationship of each of the addresseswithin group 310. The address of the first pager of the group of Mpagers to be addressed is designated address 1 and is transmitted firstin group 310 as shown. The pager to which address 1 corresponds isdesignated AB-1 for reference. The address of second pager of the groupof M selected ack-back pagers is designated address 2 and is transmittedimmediately following address 1. The pager to which address 2corresponds is designated pager AB-2. This process of addresstransmission continues sequentially in the same fashion until all of theaddresses of the group of M pagers are transmitted ending with addressM, the address of the last or M'th pager in group 310. The pager towhich address M corresponds is designated pager AB-M. A non-ack backpager AB-3 is shown addressed in the block of M pages as will bedescribed later in the discussion of FIG. 4H.

In one embodiment of the invention, the duration of time during whichpreamble signal 300 is transmitted, namely T1, is approximately equal to10 msec. Those skilled in the art will appreciate that T1 may havevalues greater than or less than 10 msec providing T1 is sufficientlylong to permit the ack-back receivers 121, 122 . . . P to synchronize tothe paging signals transmitted by central station 110. Apparatus forsynchronizing paging receivers to paging signals is well known to thoseskilled in the art and is included in ack-back pagers 121, 122 . . . P.

For purposes of example, the time duration T2 of the group 310 ofaddresses is selected to be approximately equal to 1 sec. Those skilledin the art will appreciate that T2 may actually be greater or less than1 sec depending upon the number of paging addresses M selected to be inthe group 310 and the rate of transmission of the digital datacomprising such paging addresses. The selection of the time period T2 inthis example should not be taken as in any way limiting the invention.To reiterate, the particular pagers of the population P which areaddressed in address block 310 are designated as pagers AB-1 (the firstpager to be addressed), pager AB-2 (the second pager to be addressed) .. . pager AB-M (the last pager addressed of the group of M pagers).

After transmission of the group of M addresses, central station 110transmits a reference carrier signal at a frequency F_(RX) at 320 duringa time interval T3 following time interval T2. Subsequent totransmission of reference carrier 320, central station 110 sequentiallytransmits the 20 paging messages corresponding to the 20 pagingaddresses of address group or block 310. More specifically, these M or20 data messages are sent as a group or block 330 of messages. Each ofthe M messages in block 330 bears a predetermined relationship to theorder of the pager addresses in block 310. For example, in oneembodiment of the invention and as shown more clearly in FIG. 4C,message block 330 includes message 1 data followed in time by an end ofmessage (EOM) field. The EOM field of message 1 is followed sequentiallyin time by the message 2 data which is in turn followed by another EOMfield. The process of sending the respective messages 3, 4, etc. withinmessage block 330 continues until message M is transmitted followed by arespective EOM field as shown in FIG. 4C.

In the embodiment of the invention described above, the predeterminedrelationship between the sequence of messages transmitted in messageblock 330 and the sequence of pager addresses transmitted in addressblock 310, is conveniently selected such that address 1 is firsttransmitted in block 310 and the message 1 corresponding to such address1 is transmitted first in the later following message block 330occurring during time slot T4. To illustrate this predeterminedrelationship further, address 2 is transmitted second, that isimmediately after address 1 in address block 310. Correspondingly, inthe later following time slot T4, message 2 is transmitted second, thatis, immediately following message 1's EOM field. The same relationshipexists between the remaining addresses in block 310 and messages inblock 330.

The invention, however, is not limited to the particular predeterminedrelationship described above between the sequence of pager addresses inaddress block 310 and corresponding messages in message block 330. Forexample, in another embodiment of the invention, the sequence of pageraddresses would remain as illustrated in FIG. 4B with address 1 beingsent first followed by address 2 and so forth until address M istransmitted completing the block. However, the sequential order in whichthe messages in message block 330 are transmitted in such embodiment maycommence with transmission of message M first followed by message M-1(or message 19) followed by message M-2 (18) and so forth until message1 is finally transmitted at the end of message block 310. (EOM fieldsare still situated between messages.) What is important here is that apredetermined relationship exists between the order in which the pagingaddresses are transmitted in address block 310 to the order in which thepaging messages are transmitted in message block 330 so as to permitacknowledge back pagers AB-1, AB-2, . . . AB-M to match a particularmessage within block 330 to a respective paging address of block 310.This enables a particular pager to determine which of the 20 pagingmessages in block 330 is intended for it, as will be discussedsubsequently in more detail. Although examples have been discussed abovewherein the predetermined relationship between the order of the pageraddresses of address block 310 and the paging messages of message block330 are both ascending, and in the other example ascending/descending,those skilled in the art will appreciate that an arbitrary relationshipbetween the paging addresses on block 310 and the paging messages ofblock 330 may also be selected as long as this predetermined knownrelationship is programmed into acknowledge back pagers 121, 122 . . .P.

A reference carrier exhibiting a frequency of F_(RX) is generated duringa period of time T3 subsequent to the end of transmission of the pageraddresses in address block 310. In one embodiment of the invention, T3is equal to approximately 70 msec. Those skilled in the art willappreciate that T3 may be longer or shorter than 70 msec providing thereference carrier shown at 320 exhibits a time duration sufficientlylong to enable frequency determining circuitry, later described, inack-back pagers 121, 122 . . . P to determine the frequency of referencecarrier 320.

FIG. 4D is a time vs. event diagram of the status of receiver 230 incentral station 110. Subsequent to time period T4, receiver 230 atcentral station 110 is turned on to receive ack-back signals from the 20pagers in the group of M during a time period T5. Each of the group of Mack-back message signals transmitted by the respective ack-back pagersin the group of M are on a different respective frequency sub-bandwithin a common frequency channel as will be discussed in more detailsubsequently. Receiver 230 is thus capable of distinguishing anddecoding message signals on each of the 20 or M different sub-bandfrequencies. The configuration and operation of receiver 230 isdiscussed in more detail later.

FIG. 4E is a time versus event diagram for the status of ack-back pagerAB-1, that is, the first addressed pager of the group of M pagers. FIG.4E is drawn to the same time scale as FIG. 4A. During the T1 timeinterval, pager AB-1 receives the preamble at 340. During the followingtime period T2, pager AB-1 receives and decodes address 1, which in thisexample is the address of pager AB-1. It is noted that prior toreception of the preamble at 340, pager AB-1 is in a "sleep" or "batterysaver" state That is, prior to such T1 time period, pager AB-1 and theother pagers of the population of P pagers, have several of their powerconsuming circuits turned off or placed in low power consumption states.Those skilled in the art are already familiar with the powering down ofradio pager circuits in order to achieve battery saving and thus exactlywhich circuits in the pager are powered down, and the degree to whichthey are powered down, are not discussed here in detail. What isimportant, however, is that the ack-back pagers of the population of Ppagers are placed in a "battery saving" state or "sleep state" duringprescribed periods of time such as that mentioned above and which willbe later specified.

When pager AB-1 receives the preamble 340 during time period T1, pagerAB-1 is switched from a battery saving state to a fully operationalstate such that pager AB-1 is capable of receiving informationtransmitted thereto. That is, subsequent to reception of the preamble at340, pager AB-1 is fully turned on such that pager AB-1 receives anddecodes its address at 350 at the beginning of the T2 time period. Inone embodiment of the invention, pager AB-1 conveniently returns to the"sleep state" for the remainder of the T2 time period during which pageraddresses are transmitted. Prior to receiving the reference carrierF_(RX) at time period T3, pager AB-1 is returned from the "sleep state"to the fully operational state. Upon reception of the reference carrier,F_(RX) at 360, pager AB-1 determines the frequency of such carrier in amanner described in more detail subsequently.

Referring to FIG. 4E, in conjunction with 4C, it is seen that themessage 1 transmitted during time period T4 at 370 is received by pagerAB-1 at 380 as shown in FI.. 4E. Pager AB-1 receives message 1 at 380and matches message 1 to address 1. That is, by means later described inmore detail, pager AB-1 is programmed to determine that message 1 is theparticular message of the group of M messages which is intended forpager AB-1. Subsequent to reception and display of message 1 at 380 asshown in FIG. 4E, the user of pager AB-1 indicates his or her responseto message 1 during a time period T6 at 385. Time period T6 is not drawnto scale with respect to the other time periods discussed. Time periodT6 is sufficiently long to permit indication of a response by the pageruser. Subsequent to time period T6, pagers AB-1, AB-2 . . . AB-Msimultaneously transmit acknowledge back signals on respective frequencysub-bands (subchannels) back to central station 110 as at 390 during atime period T5. Subsequent to the ack-back transmission at 390, pagersAB-1, AB-2 . . . AB-M are placed in the "sleep state" until awakenedagain by a preamble as at 340. In an alternative embodiment of theinvention, ack-back pagers AB-1 . . . AB-20 reply back automaticallywithout action by the pager user. In such an embodiment, prior to beingpaged, the user preselects a reply already stored in the pager or keysinto the pager a predetermined message which the pager uses as the ackback reply when it is later addressed by central station 110. Forexample, the ack-back pager user selects a "not available" response orotherwise keys into the pager a "not available" response when the pageruser wishes to inform callers into central station 110 that the pageruser is not taking any calls currently. Clearly, the reply data may beprovided to the ack-back pagers in many different ways. In the case of auser selectable response already programmed into the pager, time periodT6 can be arbitrarily short, that is just sufficiently long enough topermit transmission of such a selectable response whose length ispredetermined and known to the microcomputer 150 in central station 110.

FIG. 4F is a time versus event diagram of the status of ack-back pagerAB-2, that is, the second pager addressed of the group of M ack-backpagers. Pager AB-2 receives the preamble at 340 and then switches from a"sleep state" to a fully turned on state. Pager AB-2 receives address 1(the address of pager AB-1) at 350. Pager AB-2 decodes such address 1 at350 and determines that the decoded address is not its own address. At400, pager AB-2 receives its own address, namely address 2. Pager AB-2decodes and determines that address 2 is its own address. As with pagerAB-1 of FIG. 4E, pager AB-2 of FIG. 4F goes to the "sleep state" for theremainder of the T2 time period. Pager AB-2 "wakes up" in time forreception of the reference carrier F_(RX) at 360 during time period T3.As seen by examining FIG. 4F in conjunction with FIG. 4C, pager AB-2receives the AB-1 page data transmitted at 370 within time period T4. Asexplained in more detail subsequently, pager AB-2 determines that theAB-1 message data is not a match. That is, pager AB-2 determines thatthe pager AB-1 message data (message 1) is not intended for pager AB-2.After the end of message (EOM) marker following message 1, pager AB-2receives the AB-2 message data (message 2) at 410 within time period T4.Pager AB-2 determines that the message 2 data at 410 is a match and thatsuch message 2 data is intended for AB-2. The message 2 data is thendisplayed to the user of pager AB-2 who indicates an acknowledge backresponse to pager AB-2 during time period T6 at 415. During thesubsequent time period T5, the acknowledge back message is sent tocentral station 110 on a second frequency sub-band different from thefirst frequency sub-band on which pager AB-1 transmits. Subsequent totransmission of the acknowledge back response at time period T5, pagerAB-2 is caused to go to sleep.

FIG. 4G is a time versus event diagram of the status of ack-back pagerAB-M, the last of the group of M pagers to be addressed. Pager AB-Mreceives the preamble at 340 to switch it from a "battery saver state"to a fully operational state. Pager AB-M then receives the 19 addressesof the other pagers in the group of M, such as at 350 and 400 untilfinally pager AB-M receives and decodes its own address at 420. PagerAB-M is thus signaled that a message for it will be transmittedmomentarily. Pager AB-M receives the reference carrier signal F_(RX) at360. Referring to FIG. 4G in conjunction with FIG. 4C, it is seen thatpager AB-M receives message 1, message 2 . . . message M-1 anddetermines that all of these messages are not matches. That is, suchpage data messages are not intended for AB-M. Pager AB-M receives thepage data message M transmitted at 430 (FIG. 4C) and received at 440(FIG. 4G) within time period T4. Pager AB-M determines that such messageM at 440 is intended for pager AB-M and displays the contents as suchmessage M to the pager user. During time period T6 at 415, the pageruser supplies ack-back pager AB-M with an acknowledge back response.During the subsequent time period T5, pager AB-M sends such acknowledgeback response back to the central station 110 on a frequency sub-band Mat 450 different from the frequency sub-bands on which the remainingack-back pagers AB-1, AB-2 . . . AB-(M-1) transmit. Subsequent to thetransmission of the ack-back response at 450 during time period T5,pager ABM switches to the "sleep state".

One embodiment of the invention accommodates the situation where one ormore of pagers within the group of M pagers are not ack-back pagers. Forexample, it will be assumed that pager AB-3 is not a pager withacknowledge back capability, but rather is an alphanumeric display pagerwhich operates as shown in the time versus status diagram of FIG. 4H.Non ack-back pager AB-3 receives a preamble at 340 which causes pagerAB-3 to switch from a "sleep state" to a fully operational state.Subsequent to reception of the preamble at 340, non ack-back pager AB-3receives address 1 at 350 and address 2 at 400 during time interval T2.In this particular example, it is assumed that pager AB-3 is the thirdpager addressed within time interval T2. That is, address 3 is theaddress which corresponds to pager AB-3. Pager AB-3 receives address 3within time interval T2 at 460 as shown in FIG. 4H. Pager AB-3 decodesaddress 3 and determines that pager AB-3 has been paged and that a pagedata message will be transmitted to it shortly. Non ack-back pager AB-3is activated to an "awake state" during time interval T4. Pager AB-3then locates the particular AB-3 page message which is intended for itwithin time period T4. That is, since the predetermined relationshipbetween the order of the page messages transmitted within time period T4is known by pager AB-3 with respect to the order of the addressestransmitted within a time period T2, pager AB-3 locates or matches theAB-3 page data message at 470 in a manner similar to that employed bythe remaining pagers within the group of M. For example, in thisembodiment of the invention, since pager AB-3 was the third pager to beaddressed in the group of M pagers, pager AB-3 will expect its messageto likewise be third in the sequence of messages with message block 330(FIG. 4A) or more specifically at 470 of FIG. 4H. Once message 3 is soselected, pager AB-3 displays message 3 to the pager user. In thisparticular embodiment, the pager user does not have the option totransmit a response back to the central station 110. Thus, non ack-backpager AB-3 is switched to a "sleep state" after the AB-3 messagecorresponding thereto has been received

FIG. 4I is a time versus event diagram of the status of an unpagedack-back pager of the population of ack-back pagers 121, 122, . . . P.That is, FIG. 4I illustrates what occurs when an ack-back pager receivesand decodes addresses which do not correspond to the unique address ofsuch unpaged pager. More specifically, the unpaged pager, which isreferred to as pager AB-U, receives the preamble signal at 340 andswitches from a "sleep state" to a fully operational state. Pager AB-Uthen proceeds to receive a group of M or 20 pager addresses at 480during time interval T2. Pager AB-U fails to find its address withinthat group of M addresses. Thus, after time period T2, pager AB-Ureturns to the "sleep state" where it will remain for a predeterminedperiod of time. Alternatively, at the end of address block 480, a "go tosleep" signal can be transmitted to all pagers which did not receive avalid address to cause such pagers to enter the sleep state. FIG. 4Ialso represents the time versus event status of an unpaged non ack-backpager.

FIG. 5 is a flow chart of the control program which is resident in ROM170 of central station 110. This control program controls the operationof microcomputer 150 in the manner which follows. The flow chart of FIG.5 summarizes the operation of central station 110 which was describedabove in the discussion of the signaling protocol illustrated in FIG's.4A-4I. In accordance with block 500 of the flow chart of FIG. 5,microcomputer 150 is subjected to a power-on reset when it is turned on.That is, system variables are initialized at that point in time. Forexample, M, which is the number of ack-back pagers in a particular groupis initialized at a predetermined number, for example 20. Additionally,a message counter variable, I, is initialized at a value of 0 in block500. Once initialized, central station 110 is ready to accept messagesfrom telephone callers into interface 140 or from a system operator atkeyboard 160 as per block 510. When a message for a particular pageruser is input into central station 110, such message is stored in RAM180 together with indicia of the particular pager for which such messageis intended as per block 520. Such message is counted by incrementingthe message counter variable I by the quantity 1 as per block 530.Microcomputer 150 then makes a determination as to whether the number ofmessages which have been collected and stored in memory is equal to M or20 in this example. That is, as per decision block 540, microcomputer150 determines whether message counter I equals M. If the messagecounter I does not equal M, which signifies that a group of M messageshave not yet been fully collected, then flow continues to block 545where a determination is made whether or not a time out of T0, forexample T0=10 sec, has been exceeded. If the time out has not beenexceeded, then flow continues back to input block 510 to await input ofyet another message. If in block 545 it determined that the time out hasbeen exceeded, then a preamble signal is transmitted at block 550. Thistime out feature is provided so that the microcomputer 150 will not haveto wait for long periods of time for a queue of M messages to becollected prior to transmitting such messages. If prior to expiration ofthe time out, it is determined that message counter I does equal M atblock 540, then transmission of the preamble signal is commenced atblock 550.

Microcomputer 150 then looks up and retrieves from memory the addresseswhich correspond to each of the group of M pagers as seen at block 560.The addresses within such group of M pagers are sequentially transmittedin a predetermined order, for example, "first in last out" or "first infirst out", as per subsequent blocks 570 through 610. More specifically,counter I is reset to 1 and now functions as an address counter as perblock 570. Address I is retrieved from memory as per block 580. That is,in the first time through the loop starting at 580, since I=1, address 1is retrieved from memory. That is, microcomputer 150 looks up theparticular pager address which corresponds to the pager for whichmessage 1 is intended. Address I is then transmitted as per block 590.At decision block 600, microcomputer 150 makes a determination ofwhether or not all M addresses of the group of M addresses correspondingto the M messages have been transmitted. This is determined bymicrocomputer 150 calculating whether or not I is equal to M. If addresscounter I is not equal to M, then all 20 addresses have not beentransmitted and I is then incremented by 1 as per block 610. Flow thencontinues back to block 580 at which the next address of the group ofM=20 addresses is retrieved from memory. This process continues untilI=M at block 600 which signifies that all 20 addresses have beenretrieved and sequentially transmitted as a group. Program execution issuspended by block 605 until the beginning of time interval T3 at whichtime flow then continues to block 620 where reference carrier F_(RX) iscaused to be transmitted. Then, program execution is again suspended byblock 625 until the beginning of time interval T4.

Counter I is then reset to I=1 as per block 630. Counter I is nowemployed as a message counter again in the subsequent portion of theflow chart of FIG. 5. Message I is retrieved from memory at block 640.The first time through the loop starting at block 640, I is equal to 1and thus message number 1 is retrieved at block 640 the first timethrough such loop. Message I, or in this case message 1, is thentransmitted by central station 110 as per block 650. An end of message(EOM) marker is transmitted immediately subsequent to message 1 to markthe end of such message as per block 660. A determination is then madeat decision block 670 as to whether or not all of the messages in thegroup of M messages have been retrieved from memory and transmitted.This is accomplished by microcomputer 150 making a determination as towhether I is presently equal to M. If microcomputer 150 finds that I isnot yet equal to M, then I is incremented by 1 as per block 680 and flowcontinues back to retrieve message block 640. The next message, forexample message 2, is then retrieved from memory as per block 640.Message 2 is then transmitted as per block 650 and followed by an end ofmessage (EOM) marker as per block 660. This process continues untilfinally all M messages have been transmitted followed by respective EOMmarkers. It is thus seen that the M messages are transmitted as amessage group.

From the flow chart of FIG. 5, it will be observed that the group ofmessages transmitted as per block 640 through 680 bears a predeterminedorder relationship with respect to the order of the transmission of theaddresses of the corresponding group of M addresses as per blocks 570through 610. That is, in this particular example address 1 was firsttransmitted, followed by address 2 and so forth up to address M. In thisexample, the transmission of the group of M messages occurs in the sameorder as the group of addresses. That is, message 1 corresponding to thefirst address is first transmitted followed by message 2 whichcorresponds to the second address and so forth up to message M whichcorresponds to the M'th addressed pager. Other predeterminedrelationship orders are possible between the order of transmission ofthe messages of the group of M messages and the order of the group of Maddresses as has been discussed earlier. What is important, is that suchpredetermined relationship between the message order and the addressorder is known and is programmed into the ack-back pagers as isdiscussed later in more detail.

After it is determined that the transmission of the group of M messagesis complete as per block 670, flow continues to block 690 at whichcentral station 10 pauses to permit the ack-back pager users which havereceived messages to key an appropriate response into their ack-backpagers for transmission subsequently back to central station 110. Forexample, such ack-back pagers may include a keyboard or a switch that istoggled by the message recipient to signify a yes or a no. It will beappreciated that it will take significantly less time for a user totoggle one key to indicate a predetermined response, for example a yesor a "canned message" (for example, I will call you back), than it wouldtake for a user to key in a response on a keyboard or keypad situated onthe pager. However, such keyboard or keypad embodiments of the ack-backpager herein are considered to be within the scope of the invention inthat they provide alternative ways of indicating the user's response tothe ack-back pager. After pausing to permit the addressed pager users tokey in their responses, central station 110 simultaneously receives Mack-back signals from a group of M addressed pagers as per block 700.These ack-back responses are then provided to the appropriatecorresponding callers via telephone interface 140. Flow then continuesback to block 510 to permit other paging messages to be input intocentral station 100.

FIG. 6 is a block diagram of one of ack-back pagers 121, 122 . . . P.,namely ack-back pager 121. In one embodiment of the invention, ack-backpagers 121, 122 . . . P transmit acknowledge back signals on the sameradio frequency as that on which central station 110 transmits althoughthis is not necessarily a requirement of the system. That is, otherembodiments of the invention are contemplated wherein the ack-backpagers transmit ack-back signals at frequencies other than within thespectrum of the paging channel employed by central station 110. However,in the present embodiment, circuitry is included within such ack-backpagers to enable the pagers to accurately tune to and transmit ack-backsignals at different sub-bands within the same paging channel spectrumas that employed by central station 110 for transmission of pagingsignals. More specifically, each of ack-back pagers 121, 122 . . . P iscapable of transmitting ack-back signals on a plurality of M differentsub-bands within the paging frequency channel on which central station110 and the ack-back pagers transmit and receive. All of the ack-backpagers within a particular group of M addressed ack-back pagerssimultaneously transmit acknowledge back signals back to central station110 during a time period occurring after such group of M ack-back pagersare addressed and are sent respective messages. To permit suchsimultaneous transmission of ack-back signals on M different frequencysub-bands via frequency division multiplexing (FDM), it has been foundthat pagers 121, 122 . . . P must be able to tune to each of the Mdifferent sub-bands with extreme accuracy in frequency. The subsequentlydescribed frequency control circuitry within ack-back pager 121 permitssuch accuracy in sub-band frequency tuning. An example of one singleconversion receiver which is adaptable to accommodate the aforementionedfrequency control circuitry in accordance with the present invention isthe Motorola Sensar series display pager as described in the publication"Sensar" Series - Display GSC Radio Pagers, Motorola Publication No.68P81038C75-A Which is incorporated herein by reference.

Ack-back pager 121 includes a transmit/receive antenna 800 exhibiting anappropriate size and geometry to permit transmission and reception ofradio frequency signals on the radio frequency paging channel on whichcentral station 110 transmits and receives. Antenna 800 is coupled to acommon port 810A of a transmit receive switch 810. Transmit/receiveswitch 810 includes a receive port 810B and a transmit port 810C inaddition to the above mentioned antenna input port 810A. Switch 810includes a control input 810D as shown in FIG. 6. When an appropriatecontrol input signal is supplied to control input 810D, transmit/receiveswitch 810 couples antenna port 810A to receive port 810B to place pager121 in the receive mode. Alternatively, pager 121 is placed in thetransmit mode when an appropriate control signal is supplied to controlinput 810D such that transmit receive switch 810 couples the antennainput port 810A to transmit port 810C. These control signals aresupplied to control input 810D by microcomputer 820. One microprocessorwhich may be employed as microcomputer 820 is the model MCC1468705G2manufactured by Motorola, Inc.

Receive port 810B of switch 810 is coupled to the input of a radiofrequency amplifier 830. It is noted that the frequency of the radiopaging channel on which central station 110 transmits is defined to beF_(RX), for example, 150 MHz. Thus, the radio frequency paging signalswhich reach ack-back pager 121 and which are provided to amplifier 830exhibits a frequency of F_(RX) or 150 MHz. Amplifier 830 amplifies theradio paging signals from central paging station 110 and provides suchamplified signals to the input of a bandpass filter 840. Filter 840 istypically of the preselector type which filters off any undesiredsignals adjacent the paging channel frequency.

The output of filter 840 is coupled to an input 850A of a two inputmixer 850. Mixer 850 includes inputs 850A and 850B and an output 850C. Alocal oscillator 860 which oscillates at a frequency of F_(LO) iscoupled via an amplifier 870 to mixer input 850B. Mixer 850down-converts the filtered RF paging signal at frequency F_(RX) theretoby mixing such signal F_(LO) signal. In this manner, the down convertedRF signal generated at the output 850C of mixer 850 is at anintermediate frequency of F_(RX) -F_(LO) which is defined to equalF_(C).

Mixer output 850C is coupled to the input of an intermediate frequency(IF) amplifier 890 which amplifies the down-converted RF paging signals.The output of IF amplifier 890 is coupled to a count input 820A ofmicrocomputer 820 to determine the down-converted reference carrierfrequency F_(C) as later described. The output of IF amplifier 890 isalso coupled to the input a demodulator 900 which demodulates thedown-converted RF paging signals provided thereto. That is, demodulator900 separates the preamble, address, and message signals from thecarrier wave on which they were transmitted by central station 110. Thedata signals thus resulting are provided to microcomputer input 820B viaa connection to demodulator 900 as shown in FIG. 6. Such data signalsinclude preamble, address, and message signals. Microcomputer 820 ofpager 121 decodes the address signals provided at data input 820B andcompares the incoming decoded page addresses with the predeterminedunique address of such pager 121 which is stored in a code memory 910.Code memory 910 is typically an electronically erasable programmableread only memory (EEPROM) such that unique pager address codes areeasily assigned and programmed into each of ack-back pagers 121,122 . .. P. As seen in FIG. 6, memory 910 is coupled via a buss to a memoryport 820C of microcomputer 820. When microcomputer 820 determines thatone of the addresses in a received group of M pager addressescorresponds to the unique address of such pager 121, then microcomputer820 decodes the following group of M messages. Microcomputer 820 selectswhich of such messages is intended for pager 121.

In a known fashion, microcomputer 820 generates appropriate outputsignals which are supplied via linear support module 920 to audio module930 and speaker 940 to alert the pager user that a message has beenreceived. The selected message is stored in a random access memory (RAM)950 which is coupled via a bus to microcomputer memory port 820D. Aliquid crystal display module 960 is coupled to the display output 820Eof microcomputer 820 such that the selected message received by pager121 can be displayed for viewing by the pager user. Alternatively, thepager user can recall the page message from memory 950 subsequent to thealert signal for viewing later at a more convenient time. A clockcircuit 970 is coupled to a clock input 820F of microcomputer 820. Clock970 provides microcomputer 820 with a reference time base.

A user reply input device 980 is coupled to a data input port 820G ofmicrocomputer 820 as shown in FIG. 6. In one embodiment of theinvention, the user reply input device 980 is a four position switch,the positions of which are respectively designated as choices A, B, C,and D. By preagreement between the pager user and the pager caller, eachof choices A, B, C, and D is agreed to have a predetermined meaning. Forexample, choice A when selected by the pager user could be a "Yes"response to the caller's message. Choice B could be "No" response.Choice C is a "Maybe" response and Choice D is a "Cannot Reply Now"response. Those skilled in the art readily appreciate that the output ofsuch a four position switch when used in input device 980 is readilyconverted to a digital signal which is supplied to data input port 820Gfor processing by microcomputer 820. Alternatively, a 2 position orYES/NO switch could be employed in user input device 980.

It is noted that user reply input device 980 is not limited to themulti-position switch which was discussed above. Rather, other inputdevices, for example, a keyboard or other key entry devices may beemployed as user input device 980 in other embodiments of the inventionto generate reply data.

The reply data is then transmitted back to central station 110 by pager121 during acknowledge back reply field 390 as shown in the acknowledgeback protocol shown in FIG. 4E. The paging channel centered aroundfrequency F_(RX) is divided into M different sub-channels. Each pager ofthe group of M ack-back pagers which were addressed now respond backsimultaneously as a group during the appropriate acknowledge back field.Each of the M pagers of the group responds on a different frequencysub-band within the group of M sub-bands. In one embodiment of theinvention wherein M=20, the paging channel is divided into 20 differentfrequency sub-channels or sub-bands which are centered around afrequency F_(RX) and are separated by sub-channel spacings ofapproximately 1 kHz. That is, each of the 20 sub-bands, designatedsub-bands 1-20, is offset 1 kHz with respect to each other as shown inthe table of FIG. 7. The table of FIG. 7 shows each of pagers AB-1, AB-2. . . AB-20 of a group of M addressed pagers and frequency informationwith respect to the respective sub-channels or sub-bands on which suchpagers acknowledge back or respond. For example, in one embodiment ofthe invention wherein the center of the paging channel is at a frequencyF_(RX) equal to 150 MHz, pager AB-1 of the group of M addressed pagersacknowledges back on a frequency of 149.9905 MHz which corresponds to anoffset, F_(D), of -.0095 MHz with respect to the F_(RX) center channelfrequency. In a similar fashion, the pager of the group of M addressedpagers which is designated as pager AB-2 acknowledges back on a secondsub-band having a frequency of 149.9915 MHz which corresponds to anoffset, F_(D), of -.0085 MHz with respect to the F_(RX) center channelfrequency. Continuing on with pagers AB-3, AB-4 . . . AB-20, suchremaining pagers respond back on the different subchannels specified bythe frequencies and offsets shown in the table of FIG. 7.

Each of the group of M pagers designated AB-1, AB-2 . . . AB- 20, and infact all of the pagers of the population of P acknowledge back pagersare capable of acknowledging back on any one of the M differentfrequency sub-bands. That is, the control program stored within memory910 is capable of directing microcomputer 820 and associated frequencysynthesis circuitry later described to transmit acknowledge back signalson a selected one of the M or 20 different sub-bands.

In more detail, such frequency synthesis circuitry includes a voltagecontrolled crystal oscillator 1018 (VCXO) which oscillates at afrequency equal to one ninth of the ack-back signal transmit frequency,F_(TX), under the control of crystal 1020 and varactor 1022.Microcomputer 820 supplies a 12 bit binary number D from output port820M to input port 1014A of a 12 bit digital to analog (D to A)converter 1014. The computation of D will be described more fully later.An analog DC voltage proportional to D appears at output 1014B and issupplied to input 1022A of varactor 1022 which in turn exhibits a changein capacitance proportional the DC voltage applied thereto. The changein capacitance due to the range of numbers D warps the output frequencyof VCXO 1018 over an approximately 2.5 KHz range at the crystalfundamental frequency (150 MHZ/9) in this embodiment. After the VCXOoutput signal is processed by two triplers 1028 and 1036, the frequencyrange at the output of tripler 1036 of the F_(TX) signal is 22.5 KHz (or2.5 KHz×9) centered at 150 MHz. This frequency range is sufficientlywide to include all of the sub-band frequencies in the sub-bandfrequency look-up table of FIG. 7. which will be discussed later. Thisfrequency range also includes sufficient range to accommodate thetolerances of crystal 1020 as well.

In yet further detail, frequency tripler 1028 triples the frequency ofoscillator 1018 to approximately 50 MHz. The output of tripler 1028 iscoupled to a phase modulator 1032 which adds either 0 degrees or 60degrees of phase delay to the 50 MHz signal depending on whether a "0"or "1", respectively, is provided to modulator input 1032A. It is seenthat modulator input 1032A is supplied acknowledge back data via aconnection to the reply data output port 8201 of microcomputer 820. Thatis microcomputer 820 supplies acknowledge back data from output port8201 to phase modulate the acknowledge back transmit signal in modulator1032. The modulated output signal of modulator 1032 is coupled tofrequency tripler 1036 which generates an output signal which is theacknowledge back transmit signal or F_(TX) signal at a frequency (F_(TX)) of 150 MHz plus or minus the appropriate offset frequency associatedwith the sub-band frequency selected for transmission of the ack-backsignal. The aforementioned 60 degrees of phase modulation is likewisetripled to 180 degrees. Thus the output of tripler 1036 is a digitallymodulated phase shift keyed (PSK) signal at approximately 150 MHz.

The output of tripler is coupled via a filter 1030 to a radio frequencypower amplifier 1040 which is described in more detail later. Bandpassfilter 1030 filters any undesired signal components from the F_(TX)ack-back signal. Amplifier 1040 amplifies the filtered acknowledge backpaging signals provided thereto up to a signal level appropriate fortransmission back to central station 110. The output of amplifier 1040is coupled to the transmit port 810C of transmit/receive switch 810.

A sample of the F_(TX) signal at the output of filter 1030 is providedto the input of filter 840 as shown in FIG. 6. This is accomplished bycoupling a small capacitor between the output of filter and the input offilter 840. In this manner the output of filter 1030 is lightly coupledto the input of filter 840 such that a relatively low level version ofthe ack-back signal at F_(TX) is fed back to filter 840 and thecircuitry beyond. As will be shown later, this fed back signal is usedby the frequency control algorithm of microprocessor 820 to synthesizethe F_(TX) ack-back transmitter signal nearly precisely on the selectedsub-band frequency.

It is noted that there is a predetermined relationship between theparticular sub-band frequency on which each of ack-back pagers AB-1 -AB-20 responds to either the order of each pager's particular addresswithin the group of M pagers or the order of each pager's particularmessage within the group of M pagers. From the earlier discussion, itwill be recalled that the order of the messages within a group of Mmessages bears a predetermined relationship to the order in which theaddresses for such messages were transmitted in the correspondingaddress group. The relationship between the selection of frequencysub-bands for ack-back transmission and the order of transmission of theM addresses or M messages is established to enable microcomputer 150 incentral station 110 to determine which ack-back signal sub-bandtransmission corresponds to which acknowledge back pager address of thegroup of M pagers.

For example, assuming that pager AB-1 in the table of FIG. 7 is thefirst ack-back pager of the group of M pagers to be addressed or receivea message, then, ack-back pager AB-1 responds back on a sub-channel orsub-band frequency designated sub-band 1 which corresponds to thefrequency and offset noted in Table 1. Assuming that pager AB-2 in thetable of FIG. 7 is the second pager of the group of M pagers which isaddressed or sent a message, then, pager AB-2 acknowledge back onsub-band number 2 which corresponds to a frequency and offset shown inthe table of FIG. 7. To continue this example, assuming that pager AB-20is the twentieth pager of the group of M pagers to be addressed orreceive a message, then pager AB-20 acknowledges back on a sub-bandfrequency 20 which corresponds to the frequency and offset shown in thetable of FIG. 7. Although each of pagers AB-1, AB-2 . . . AB-20 respondsback on the different respective sub-bands 1-20 noted in FIG. 7, all ofsuch pagers respond back simultaneously in a common time slot or fieldas already described.

It is noted that other predetermined relationships between the ack-backsub-band order and the order in which the addresses or messages weretransmitted to the group of M pagers may be employed. That is, althoughin the example above, the order of the M addresses (or M messages) andthe corresponding order of the M sub-bands are both ascending, inanother embodiment of the invention in which the order of the addressesof the group of M pagers AB-1 . . . AB-20 is the same as the priorexample (ascending), the order of the acknowledge back sub-bands isreversed as compared to the prior example (descending). That is, pagerAB-1 responds back on sub-band 20; pager AB-2 responds back on sub-band19 . . . and pager AB-20 respond back on sub-band 1.

Also, as mentioned briefly earlier in this document, alternatively inanother embodiment of the invention, the relationship between the orderin which pager addresses or messages were received by the group of Mpagers and the order of assignment of sub-bands for ack-back to such Mpagers can be arbitrary. What is important is that a predeterminedrelationship exists between the order of assignment of sub-bands and theorder in which the pager addresses or messages arrive at the group of Mpagers. Again, this predetermined relationship is programmed into memory170 of microcomputer 150 in central station 110 such that microcomputer150 can determine which sub-band is being used by each of the pagersAB-1, AB-2 . . . AB-20 as they acknowledge back.

An example is now presented showing how one of the AB-1, AB-2 . . .AB-20 pagers selects a sub-band frequency on which to respond andgenerates an acknowledge back signal at that frequency. For purposes ofthis example, the third pager to be addressed or receive a message inthe group of M pagers, that is pager AB-3, will be discussed. In thisexample, unlike the example of FIG. 4H, pager AB-3 is an acknowledgeback pager. After reading the message which is supplied to the display960 of pager AB-3 (such as pager 121 of FIG. 6), the pager AB-3 userindicates a reply at input device 980 as already discussed. The controlprogram in memory 910 of pager AB-3 causes microcomputer 820 therein torecognize that AB-3 is the third pager of the group M =20 pagers to beaddressed. A sub-band look up table is stored in memory 910. Thesub-channel look up table includes the appropriate frequency offset,F_(D), for each of the 20 different frequency sub-channels as shown inFIG. 7. As mentioned, microcomputer 820 of pager AB-3 determines that ithas received the third address or third message in the respectiveaddress or message group sequences. Using this information,microcomputer 820 fetches from memory the particular frequency offset,FD, from the sub-band look up table in memory 910 which corresponds tothe third sub-band or sub-band 3.

In the circuit arrangement of FIG. 6, the ack-back frequency F_(TX)equals the ninth harmonic of the V_(CXO) frequency at 1018. Such V_(CXO)frequency is voltage warped under the control of microcomputer 820.During execution of the microcomputer frequency control algorithm,microcomputer 820 first sets the 12 bit input, D, of the D/A circuit1014 to a midrange value of 2048 plus the frequency offset F_(D). Inthis particular embodiment of the invention, a D/A converter having arange of 4096 is employed as D/A circuit 1014. A value of D=2048corresponds to the center (150 MHz) of the range of sub-band frequenciesset forth in the sub-band look-up table of FIG. 7. There is nearly alinear relationship between 1 step in D and 1 unit change in theack-back transmit frequency F_(TX). To reiterate, microcomputer 820initially sets D=2048+F_(D) to drive the ack-back transmit frequencyF_(TX) approximately to the frequency of the selected sub-band.

Oscillator 1018 then exhibits an output frequency whose ninth harmonic(the F_(TX) ack-back transmit signal) is lightly coupled back to thereceive portion of ack-back pager 121 through coupling capacitor 1038.The F_(TX) transmit signal is then down-converted to produce thedown-converted transmit signal, F_(CTX). Microcomputer 820 thendetermines the frequency of the down-converted signal, F_(CTX), bycounting the frequency of such signal at the microcomputer input 820A.It is noted that previously during the time interval, T₃, microcomputer820 determines the frequency of the down-converted reference carriersignal F_(C) and stores the result in memory 950. Microcomputer 820 nowretrieves the reference carrier frequency count F_(C) and the currentF_(CTX) count from memory 950. Microcomputer 820 also retrieves theF_(D) frequency offset for the third sub-band from the sub-band looktable stored in memory 910 (assuming that the third sub-band is thesub-band which has been selected for transmission of ack-back signals)Microcomputer 820 then computes a new value of D which equals the old Dplus (F_(C) +F_(D))-F_(CTX). The new value of D is provided to the D/Acircuit 1014 via microcomputer output 820M, thus altering the controlvoltage applied to varactor 1022. The above expressed algorithm isiterated I times until the difference ((F_(C) +F_(D))-F_(CTX)) issubstantially close to zero. In equation form, D=D+(F_(C)+F_(D))-F_(CTX) or D_(NEW) =D_(OLD) +(F_(C) +F_(D)) -F_(CTX). Severaliterations I are required because the relationship between frequency andvaractor voltage is not precisely linear. In this embodiment of theinvention, it has been found that J=4 iterations are sufficient to bringthe actual F_(TX) transmit frequency of pager 121 to withinapproximately 30 Hz of the desired F_(TX) sub-band frequency. Thoseskilled in the art will appreciate that the invention is not limited tothe I=4 iterations presented above by way of example. Clearly, feweriterations will not bring the actual F_(TX) frequency as close to thedesired sub-band frequency and more iterations will bring the actualF_(TX) frequency even closer to the desired sub-band frequency. Thus,both fewer and more iterations of the algorithm than 4 are intended tobe within the scope of the invention.

The above described circuit arrangement employs both the receiversection of pager 121 and microcomputer 820 to measure the frequency ofthe down converted reference carrier at F_(C) and subsequently the downconverted actual transmitter frequency F_(CTX). It accomplishes thistask by counting the down converted F_(RX) reference carrier at F_(C)and by subsequently counting the down-converted actual transmitterfrequency at F_(CTX). The frequency control algorithm corrects theactual transmitter frequency F_(TX) by iteratively minimizing thedifference between the quantity (reference count F_(C) plus the offsetF_(D)) and the count of the F_(CTX). The algorithm drives thisdifference substantially to zero in I iterations. Thus, the frequencycontrol algorithm drives the difference ((F_(C) +F_(D))-F_(CTX))substantially to zero.

Although, a single conversion embodiment of acknowledge back pager 121is shown in FIG. 6 and described above, those skilled in the art willappreciate that double and other multiple conversion embodiments of thepager are readily adapted from this invention and are intended to bewithin its scope.

Each of pagers 121, 122 . . . P includes a threshold detector 1050coupled between the output of amplifier 890 and input 820J ofmicrocomputer 820. Threshold detector 1050 provides input 820J a logical0 when the down-converted carrier signal at F_(C) exhibits a voltagelevel less than a predetermined threshold level. However, when thesignal voltage level of the F_(C) carrier signal is equal to or greaterthan such selected predetermined voltage level, then threshold detector1050 provides a logical 1 to microcomputer input 820J. The threshold isset, for example, so that a signal at the receiver input which is 40 dBabove minimum usable receiver sensitivity will trigger thresholddetector 1050. Microcomputer 820 includes a power control output 820Kwhich is coupled to a power level control input 1040A of variable outputpower amplifier 1040. Amplifier 1040 is of the type which can assumedifferent power output levels depending upon the value of the signalprovided to 1040A. For example, in this particular embodiment, when alogical 0 is provided to input 1040A, amplifier 1040 operates ortransmits at full power, for example at approximately 1.5 watts output.However, when a logical 1 is provided to input 1040A, amplifier 1040throttles back or reduces power to a second lower power output levelwhich is approximately 40 dB less than the full power output level. Insummary, in this embodiment of the invention, when a logical 0 isprovided by threshold detector 1050 to microcomputer input 820Jindicating that a relatively low level signal is being received, thenmicrocomputer 820 generates a logical 0 at its output 820K. This causesamplifier 1040 to amplify at the first or full output power. However,when threshold detector 1050 provides a logical 1 to microcomputer input820J, indicating that a relatively high level signal is being received,microcomputer 820 then generates a logical 1 at output 820K. This inturn causes amplifier 1040 to throttle back to the second lower outputpower level. The above described variable output power level circuitarrangement aids in avoiding the situation when any one of the group ofM pagers AB-1 . . . AB-20 generates such a strong ack-back signal atcentral station 110 that such signal exceeds the dynamic range of thereceiver of station 110 and masks the ack-back signals from the otherpagers of the group of M.

Although in this particular embodiment of the invention, a two powerlevel amplifier 1040 is employed in conjunction with a single levelthreshold detector 1050, the invention may also be practiced usingthreshold detectors with more than one threshold and variable outputpower amplifiers with more than two selectable output powers. Forexample, in an alternative embodiment of the invention, thresholddetector 1050 is a three range threshold detector which determines ifthe F_(C) signal exhibits a low, medium or high signal level. Such athreshold detector conveniently employs first and second thresholds.That is, when threshold detector 1050 determines that the receivedsignal level at the pager is within a first predetermined low signallevel range (less than the first threshold), then microcomputer 820causes a three output power level amplifier, employed as amplifier 1040,to amplify at a high output first power level. When the three rangedetector 1050 detects that the received signal level is within a mediumsignal level range (between the first and second thresholds), thenmicrocomputer 820 would causes amplifier 1040 to amplify at a mediumoutput second power level. When detector 1050 determines that thereceived signal level is within a third high level range (above thesecond threshold level), then microcomputer 820 causes amplifier 1040 tofully throttle back to a third and lowest power output level. Thus, apower control circuit is provided in which the transmitted output powerof the ack-back pager varies inversely with the RF signal level of thepaging signals it receives from central station 110.

Microcomputer 820 is programmed to generate a logical 1 at port 820Lduring the period of time at which pager 121 is to transmit anacknowledge back signal back to central station 110, for example,ack-back time period 390 as shown in FIG. 4E. During all other periodsof time for which pager 121 should be in the receive mode, microcomputer820 is programmed to generate a logical 0 at port 820L. When a logical 1is generated at output 820L, indicating transmit mode, transmit/receiveswitch 810 connects antenna port 810A to port 810C thus connecting thetransmit amplifier 1040 to antenna 800. However, when a logical 0 isprovided to microcomputer port 820L, transmit/receive switch 810 couplesantenna port 810A to port 810B and receiver amplifier 830.

FIG. 8A and 8B in combination depict is a flow chart of the controlprogram stored in memory 910 which controls the operation ofmicrocomputer 820 and pager 121. A power-on-reset step is shown in block1100. Program variables are initialized at this time. The receiverportion of pager 121 is turned on and becomes synchronized with respectto the paging signals transmitted on the paging channel by centralstation 110. After becoming initially synchronized, pager 121 goes intoa "sleep mode" or battery saving mode as described earlier. When pager121 receives a preamble signal, as in block 1110, pager 121 wakes up asper block 1120. An address count variable, ADRCOUNT, is then initializedwith a value of 0 as per block 1130. A variable ADRMAX which representsthe maximum number of ack-back pagers in an ack-back group is set tohave a value of M as per block 1130. Pager 121 listens to each of theaddresses within a group of M addresses to determine if its particularaddress is received as per block 1140. For example, at block 1140, thefirst address of a group of M addresses is checked to determine if it isthe valid address for the particular pager 121. If the first address isnot the address of pager 121, then the ADRCOUNT variable is incrementedby 1 to count the number of pager addresses already received as perblock 1150. A determination is then made as to whether all of theaddresses of the group of M addresses have been processed, block 1160.If the variable ADRCOUNT is equal to M, then the address of theparticular pager 121 has not been received and such pager 121 reentersthe battery saver mode as per block 1170 after which pager 121 againpowers down and looks to determine if a preamble signal is received. Ifhowever in block 1160 ADRCOUNT is not equal to M, that is less than Msignifying that all of the M addresses of a group of M addresses havenot been received as in the present example with respect to the firstaddress of such group, then flow continues back to block 1140 wherepager 121 checks the next address in the group of M addresses forvalidity. If any address within the group of M addresses is determinedto be the address for the particular pager 121, then flow continues fromblock 1140 to block 1180 at which the variable ADRCOUNT is incrementedby 1 such that ADRCOUNT is a number which represents the order of thevalid address within the sequencing or group of M addresses.

After the group of M addresses is received by pager 121, pager 121receives and determines the frequency of the down-converted referencecarrier F_(C) as per block 1190. The signal strength of the carrierF_(C) is then determined by micro processor 820 as per block 1200.

In the following steps, the particular message within the group of Mmessages which is intended for a particular pager within the group of Maddressed pagers is matched with such pager and displayed thereon. Moreparticularly, prior to commencing to count the number of messages withinthe group of M messages as such messages are received, a message countvariable MSGCOUNT is initialized at a value of 0 as per block 1210. Thereceiving of the individual messages of the group of M messagescommences as per block 1220 at which the next message of such group isreceived. Initially, the first message of the group of M messages is the"next message" received Upon reception of a message, the MSGCOUNTvariable is incremented by 1 to count the number of messages that havebeen received as per block 1230. A determination is then made as towhether MSGCOUNT equals ADRCOUNT at block 1240. If it is determined thatMSGCOUNT does not equal ADRCOUNT, then more messages remain to bereceived in the group of M messages and flow continues back to block1220, at which the next message is received. In this example, whereinthe first message was received the first time around the loop formedbetween block 1220 and 1240, the second message is received the secondtime around such loop and the message counter MSGCOUNT is incremented at1230 accordingly. When a determination is made that MSGCOUNT equalsADRCOUNT then, the current message is displayed at block 1250. In thismanner, the particular message which was intended for a pager within thegroup of M pagers is displayed by matching the order of the occurrenceof such message in the group of M messages with respect to the order ofthe corresponding address within the group of M addresses.

Ack-back data is supplied to microcomputer 820 by the pager user as perblock 1260. The ack-back pager waits as per block 1270 for an ack-backfield (time interval) before responding back to the central station 100with the ack-back data provided by the pager user. It was discussedearlier that M different sub-bands are available in the pager of theinvention for transmission of ack-back signals. Each ack-back pagerwithin a group of M addressed pagers responds back to the centralstation 110 on a different respective sub-band based on the value of theADRCOUNT variable determined above for such pager as per block 1280. Forexample, in one embodiment of the invention, if a particular pagerwithin the group of M pagers is the fifth pager of the group to beaddressed, then such pager has an ADRCOUNT value of 5. As per the abovediscussion, the fifth message in the group of M messages corresponds tothe fifth pager addressed and is appropriately provided to the displayof such fifth pager for viewing by the pager user. In this particularpager wherein ADRCOUNT equals 5, sub-band number 5 is selected from thetable of FIG. 7 for use by such pager for transmitting its ack-backsignal. That is, the value of ADRCOUNT determines the particularsub-band which is selected for ack-back. Since in this particularexample sub-band 5 is selected, microcomputer 820 accesses the sub-bandchart of the table of FIG. 7 and looks up the frequency offset F_(D)corresponding to sub-band number 5 as per block 1290. The value ofF_(C), the down-converted carrier frequency, is then retrieved frommemory or is otherwise acquired as per block 1310.

Microcomputer 820 initially sets the input signal D to D/A converter1014 to a value of 2048+F_(D) in block 1304. Oscillator 1018 along withtriplers 1028 and 1036 are turned on in block 1308. A loop counter I isset to a value of 4 in block 1312. I represents the number of iterationsmade by the frequency control algorithm and in actual practice may begreater or lesser than 4 as previously explained. As mentioned earlierthe F_(TX) transmit signal is fed back to the receiver portion of pager121 and is down-converted to produce the down converted F_(CTX) signal.The frequency of the down converted F_(CTX) signal is counted at block1316 and is stored in memory 910. Microcomputer 820 recalculates thevalue the of D according to the relationship D=D+(F_(C) +F_(D))-F_(CTX)and provides the new value of D to D/A converter 1014. The loop counterI is decremented by 1 at block 1324. The loop counter I is tested atblock 1328. If I has not yet been decremented down to a value of lessthan or equal to 1 then flow continues back to block 1316 where thefrequency F_(CTX) is again determined. Otherwise, flow continues toblock 1340 as discussed subsequently.

A determination is then made by microcomputer 820 as to whether thesignal level of the F_(C) reference carrier is greater than theaforementioned predetermined threshold level. If the F_(C) signal levelis greater than a predetermined threshold level as determined at block1340, then the transmitter circuits of pager 121 are turned on, as atblock 1350. The ack-back data is then transmitted back to centralstation 110 at a low power level on the already selected frequencysub-band via frequency division multiplexing as per block 1360. Aftertransmission of the ack-back data, the transmitter circuits are turnedoff at block 1370 and the battery saver mode is reentered as at block1170. If, however, it is determined at block 1340 that the F_(C) carrierreference signal does not exhibit a signal level greater than thepredetermined threshold, then the transmitter circuits of pager 121 areturned on at block 1390 and the ack-back data is transmitted back tocentral station 110 at a high power level on the selected frequencysub-band via frequency division multiplexing as per block 1400. Aftersuch transmission of the ack-back data, the transmitter circuits areturned off at block 1370 and the battery saver mode is reentered atblock 1170.

From the above description, it is clear that the invention involves amethod of radio paging for employment in a radio paging system includinga central paging station for transmitting paging signals on a pagingchannel frequency F_(RX) to a plurality of remotely located acknowledgeback radio pagers. The central station transmits a reference carriersignal, that is an F_(RX) signal, at frequency FRX at selected times.The method of the invention is a method of controlling the frequency FTXon which the acknowledge back pager transmits an acknowledge backsignal. The method includes the step of receiving and down-convertingthe F_(RX) signal thus producing a down-converted F_(C) signal. Themethod includes measuring the frequency of the down-converted F_(C)signal. The method further includes selecting a sub-band from aplurality of frequency sub-bands within a predetermined range offrequencies for transmission of an ack-back signal thus determining aselected sub-band The method includes the step of determining afrequency offset F_(D) with respect to a predetermined frequency withinthe range of frequencies. The method includes the step generating aradio frequency signal substantially at a frequency (F_(C) +F_(D))-F andup-converting the radio frequency signal to a transmit frequency F_(TX)corresponding to the selected sub-band.

In summary, the foregoing describes an apparatus and method forcontrolling the transmit frequency of an acknowledge back radio pagerwhich permits the pager to transmit on a selected one of a plurality offrequency sub-bands with high accuracy.

While only certain preferred features of the invention have been shownby way of illustration, many modifications and changes will occur tothose skilled in the art. It is, therefore, to be understood that thepresent claims are intended to cover all such modifications and changeswhich fall within the true spirit of the invention.

We claim:
 1. In a radio paging system including a central paging stationfor transmitting paging signals including a group of pager addresses ona paging channel frequency F_(RX) to a group of corresponding remotelylocated acknowledge back radio pagers, said central station transmittingsaid pager addresses in a predetermined order over a first timeinterval, each acknowledge back pager capable of controlling a subchannel frequency F_(TX) on which it transmits acknowledge back signals,an acknowledge back pager of said system comprising:receiving means forreceiving said paging signal on the channel frequency F_(RX) ; measuringmeans, coupled to said receiving means, for measuring the channelfrequency of the received paging signal; selecting means for selecting asub-band frequency from a plurality of frequency sub-bands within apredetermined range of frequencies in accordance with a predeterminedrelationship based on the predetermined order in which said pager'saddress is being transmitted in said group, said selected frequencysub-band being the sub-channel frequency for transmission on anacknowledge back signal by said pager; determining means for determininga frequency offset F_(D) of the measured channel frequency correspondingto said selected sub-band frequency; transmitter means, coupled to saiddetermining means, for generating an acknowledge back signal at saidselected sub-band frequency including both the measured channelfrequency and the determined offset frequency.
 2. In a radio pagingsystem including a central paging station for transmitting pagingsignals on a paging channel frequency F_(RX) to a plurality of remotelylocated acknowledge back radio pagers, a method of controlling asub-channel frequency F_(TX) on which an acknowledge back pager of saidsystem transmits an acknowledge back signal, said method comprising thesteps of;receiving the paging signal and down-converting the channelfrequency F_(RX) thereof to produce a signal at a down-convertedfrequency F_(C) ; measuring the frequency F_(c) of said down-convertedsignal; selecting a sub-band frequency unique to said acknowledge backpager from a plurality of frequency sub-bands within a predeterminedrange of frequencies as the sub-channel frequency for transmission of anacknowledge back signal from said pager; determining a frequency offsetF_(D) corresponding to the selected sub-band frequency; generating amodulated signal at an initial frequency based on a desired frequencysetting (F_(C) +F_(D)) and up-converting the initial frequency of themodulated signal to a transmit sub-channel frequency F_(TX)corresponding substantially to said selected sub-band frequency;down-converting the frequency F_(TX) of said generated up-convertedsignal and measuring the down-converted frequency F_(CTX) thereof; andadjusting said initial frequency of said modulated signal in a directionto converge the measured frequency F_(CTX) to the desired frequencysetting F_(C) +F_(D), whereby the transmit sub-channel frequency F_(TX)of the pager's acknowledge back signal is caused to be set precisely atthe selected sub-band frequency,
 3. In a radio paging system including acentral paging station for transmitting paging signals on a pagingchannel frequency F_(RX) to a plurality of remotely located acknowledgeback radio pagers, a method of controlling a sub-channel frequencyF_(TX) on which an acknowledge back pager of said system transmits anacknowledge back signal, said method comprising the steps of;A.receiving the paging signal and down-converting the channel frequencyF_(RX) thereof to produce a signal at a down-converted frequency F_(C) ;B. measuring the frequency F_(C) of said down-converted signal; C.selecting a sub-band frequency unique to said acknowledge back pagerfrom a plurality of frequency sub-bands within a predetermined range offrequencies at the sub-channel frequency for transmission of anacknowledge back signal from said pager, said sub-channel frequencybeing unique to said pager; D. determining a frequency offset F_(D)corresponding to the selected sub-band frequency; E. generating amodulated signal at an initial frequency based on a desired frequencysetting (F_(C) +F_(D)) and up-converting said initial frequency of themodulated signal to an initial transmit sub-channel frequency F_(TX)approximately corresponding in frequency to said selected sub-band; F.down-converting said frequency F_(TX) of the generated signal togenerate a signal with a down converted frequency F_(CTX) ; G.generating the modulated signal at an adjusted frequency in a directionto converge the measured frequency F_(CTX) to the desired frequencysetting (F_(C) +F_(D)) and up-converting said adjusted signal to atransmit sub-channel frequency F_(TX) corresponding more closely infrequency to said selected sub-band; and H. repeating steps F and Guntil the frequency of the transmit signal F_(TX) is substantially equalto the frequency of the selected sub-band.
 4. In a radio paging systemincluding a central paging station for transmitting paging signals on apaging channel frequency F_(RX) to a plurality of remotely locatedacknowledge back pagers, an acknowledge back pager capable ofcontrolling a sub-channel frequency F_(TX) on which it transmitsacknowledge back signals, said pager comprising:means for receiving thepaging signal on the channel frequency F_(RX) ; means for measuring thechannel frequency of the received paging signal; controller means fordetermining a desired sub-channel frequency setting based on themeasured channel frequency and a selected frequency offset frequencyunique to said acknowledge back pager and for generating a controlsignal with an initial value substantially representative thereof;frequency synthesizer governed by said control signal to generate asignal at a frequency which is used by said pager for transmitting theacknowledge back signal thereof; and means for measuring the frequencyof said signal generated by said frequency synthesizer, said controllermeans further operative to adjust the initial value of said controlsignal in a direction to converge the measured frequency of said signalto the desired sub-channel frequency.
 5. The acknowledge back pager inaccordance with claim 4 including:means for amplifying the acknowledgeback signal of said paper prior to transmission thereof in accordancewith a controlled amplification setting; means for measuring themagnitude of the amplified acknowledge back signal; and means governedby said magnitude measurement of the measuring means to control theamplification setting of said amplifying means.
 6. The acknowledge backpager in accordance with claim 4 wherein the controller means comprisesa programmed digital controller which generates a digital controlsignal, the digital code of which represents a value of the frequencysetting; and wherein the frequency synthesizer comprises a voltagecontrolled oscillator which is digitally controlled by said digitalcontrol signal to generate the sub-channel frequency signal.
 7. Theacknowledge back pager in accordance with claim 4 including a modulatingmeans for modulating the sub-channel frequency signal, said controllermeans being further operative to govern the modulating means to modulatethe sub-channel frequency signal with the acknowledge back signal ofsaid pager.